Macrophages from the main route for host infection with the human immunodeficiency virus (HIV). Understanding the mechanisms involved int he entry of HIV into macrophages has begun to be elucidated with the discovery of the coreceptor activity of CCR5. The goal of this proposal is to define the molecular determinants involved int eh establishment of HIV infection in macrophages. to accomplish this aim, two separate studies will be performed. First, coreceptor activity of macrophages will be examined using an expression-cloning protocol. Macrophage- derived cDNAs will be transfected into a nonpermissive cell line and the expression of gene products which support infection by macrophage-tropic HIV molecular clones will be assessed. In addition, a sensitive assay for envelope-dependent cell fusion will be utilized for screening. Positive clones will be identified and standard molecular biological techniques will be used to obtain and sequence a full-length cDNA. The amino acid sequence will be predicted and analyzed for structural features that may be indicative of the role this protein plays in macrophage infection. Second, the molecular determinants of macrophage and T cell line-tropism will be assessed using CCR5/CXCR4 chimeras and a vast collection of envelope glycoprotein constructs. The chimeric molecules will be tested for the abilities to support infection, fusion, binding and Ca** flux dependent on the phenotype of viral gp 120. Together, results from these studies should contribute to the development of specific HIV inhibitors and a small animal model for HIV infection.